This invention relates to AC motor controllers for vehicles having an electrical drive. In particular it relates to the integration of an AC motor into a vehicle having a multi-speed transmission as part of the power train connecting the motor to the wheels in a system that provides for regenerative braking.
Existing AC motor systems for electric vehicles use a fixed overall gear reduction ratio generally of about 12:1. The measured rotor speed in such installations is a reliable guide for setting the desired speed of the rotating field in the stator""s field windings. For accelerating a small increase is effected in the field windings rotational speed, over the speed of rotation of the rotor. For decelerating (also called regenerative retard), a small decrease in such speed is required. Expensive, specially designed motors which can operate at very high rpms are required to attain a speed of 100 km/h in a typical AC powered vehicle when a fixed gear ratio is employed.
When an electric vehicle is equipped with a typical 4 speed manual transmission overall reduction ratios of 15:1, 8.3:1, 5.6:1 and 3.9:1 are available. At 100 km/h in 4th gear the motor only has to turn at 3660 rpm. At this speed the motor is very efficient and standard, inexpensive industrial AC induction motors can be used.
In an electric vehicle having a shifting transmission, prior to shifting the rotor will be turning at a speed that corresponds to the vehicle wheel speed, as modified by the transmission ratio. After shifting, when the rotor is mechanically reconnected to the wheels through the transmission, the rotor will be turning at a new speed, again dictated by the wheel speed and transmission ratio combined.
The shift between 1st and 2nd transmission ratios is typically a change of 45% which is too large for existing AC controllers to handle.
A mechanical shock will occur when the rotor reconnects to the wheels and readjusts to the new speed dictated by the wheels through the powertrain. But a rotor is relatively light and will conform quickly to the new speed requirement. However, an electrical mismatch can arise when this reconnection occurs. And this electrical mismatch has more serious consequences.
In an AC motor the field coil, to operate effectively, must produce a magnetic field that is rotating at a speed that corresponds to the speed of rotation of the rotor. In a synchronous AC motor, these speeds will be the same. In an asynchronous, induction AC motor, these speeds will differ by the slip that is present. But, with an allowance for slip, the rotational velocity of the stator field will xe2x80x9ccorrespondxe2x80x9d to the mechanical rotational speed of the rotor. These conditions apply when the AC motor is operating effectively.
The current waveform fed to the field coil of a variable speed AC motor must be constructed by a wave-form generating motor controller to create a rotating magnetic field. This waveform has a frequency of oscillations that will deliver a rotating magnetic field of appropriate rotational velocity within the stator coils of the motor. It is the function of an AC motor controller to deliver to the stator winding of an AC motor a field coil activating current of appropriate frequency the stator winding of an AC motor that corresponds to the rotational velocity of the rotor.
When an AC motor experiences a transmission shift, if a substantial mismatch occurs between the stator field""s rotational velocity and the rotor""s rotational speed, then there can be a severe reduction of torque. Further, electrical transients may occur that expose the AC motor control system to voltage or current spikes that require protective features and protocols to be included in such system.
As an example, when upshifting from first to second gear at say 30 km/h the rotor may be initially turning at 4150 rpm. After the shift it may be turning at only 2296 rpm. If the controller were asked to continue driving the field coil at 4150 rpm following the shift, a dangerous stall condition could arise. At best a long delay would occur before the rotor would accelerate to its proper speed. Downshifting is even worse. A downshift from second to first at 30 km/h calls for a typical change in rotor rpm of from 2296 rpm to 4500 rpm. If the field winding of the motor is still powered at the old speed of 2296 rpm following this shift, then a large negative slip condition will arise and pour considerable energy back into the batteries through the motor control circuitry. This can easily raise the instantaneous voltage applied to the electrical circuitry to a breakdown value. For example, the snubber capacitors and IGBT""s (internal gate bipolar transistors) components in a controller could fail.
The present invention addresses a means by which an AC motor controller in an electrically powered vehicle may accommodate a transmission shift without exposing the system to prejudicial electrical consequences.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended to demonstrate the principle of the invention, and the manner of its implementation. The invention in its broadest and more specific forms will then be further described, and defined, in each of the individual claims which conclude this Specification.
To ensure that a mismatch of the stator field rotational velocity for an AC motor does not arise during a gear change, it is a feature of the invention to provide a means for suspending the flow of current to the field coils of the motor when a transmission ratio shift is in progress. According to one variant of the invention the provision of current to the field coil is suspended when the rotor-to-motor speed ratio has departed from a value that is consistent with there being a mechanical connection between the rotor and the wheels. This can be established by detecting a change in such ratio that can only arise when a transmission shift is in progress.
According to another variant of the invention motor speed and wheel speed are monitored on a continuous basis. A controller then proceeds to divide the smaller wheel rpm into the larger motor rpm (or vice versa). If this value is not substantially within the range of known transmission ratios, the motor may be considered to be disengaged from the wheels. In any of these conditions, the controller should, in an abundantly cautious system, immediately suspend activation of the field coils. Alternately, combinations of such conditions may be required.
Reactivation of the field coil is only permitted to occur when the rotor has stabilized at a new rotational speed that corresponds to re-engagement of the rotor to the vehicle wheels. The excitation of the field coils is then reactivated at a rotational velocity that is within a few percent of the rotor""s measured velocity, according to the slip condition that is required, if slip is to be present. Whether positive or negative torque is to be generated within the motor is then established by input from the operator.
Reactivation thus occurs proceeding from an inactivated or unpowered state only once the rotor speed has stabilized. This avoids an undesirable mismatch between the field coil excitation velocity and the rotor speed.
The re-powering of the motor may be made subject to analogous tests to those described above. If the resultant ratio value for wheel speed vs rotor speed is close to one of the expected ratios for the transmission, and preferably, if the present measured ratio is the same, within an acceptable tolerance, as the most recently measured previous ratio, then the clutch may be considered to be engaged and the motor""s rotary speed may be considered to be reliable for control purposes. It is at this stage that the field coil may be re-excited with the appropriate rotational velocity.
An important condition for re-powering of the motor is that the measured ratio is within the range of permitted ratios. A supplementary test of comparing consecutively measured ratios, vis when two successive measures of motor and wheel rpm both yield, effectively, the same ratio value, can be added to ensure that the measured rotor rpm can be safely used as an input for control purposes, permitting waveform generation for the field coils to be restored. Circuitry to ensure this effect serves as a confirmatory engagement means.
As additional preferred features the following procedures may be applied. Wheel rpms based on measurements on only one wheel are exact when the vehicle is travelling in a straight line. When going around curves true vehicle speed is best made using two (opposite) wheel speed sensors whose results are added, then divided by two.
An alternate strategy is to use only one wheel speed sensor and allow a liberal amount or range for acceptable transmission ratios for the lower gear ratios. Since tight turning is only going to occur at low speeds, the tolerated acceptable ratios at high speeds can be, and should preferably be, more stringent. If allowed or accepted values are of too broad a range, then delays in disabling the waveform during shifts could be a problem. If too narrow, delays before resuming waveform generation following a shift could be needlessly prolonged. Skilled workmen testing the alternatives will be readily able to establish the preferred range.
For startup conditions, the vehicle wheel rpm is zero and the above wheel speed to rotor speed ratio strategy will not be effective. A divide-by-zero condition arises. In this case it is permissible for the controller to activate the field coil with an arbitrary, but low, rotational velocity. This may be allowed when first gear or reverse gear conditions are occurring, both of which have high gear reduction values, e.g. 15:1 suited for very low vehicle wheel rpms. The field coil may be activated at a rotation velocity which is a greater mismatch with rotor speed under these conditions. Attempting to start in higher ratios is not serious, other than that insufficient torque may be available for start-up.
The strategy of the invention for making successful transmission upshifting or downshifting permits the AC motor, or motor acting as a generator in regeneration mode, to be operated in combination with the transmission in its most efficient regime, its xe2x80x9csweet zonexe2x80x9d, for optimal efficiency for all but short durations during start-ups and final braking stops. It does not prevent motor breakdown due to motor overloading, such as attempting to climb too steep a hill too fast. Therefore, usual motor overloading protection measures are still required. Similarly, regenerative current-limiting provisions should be used to protect against system failure.
An important step in the procedures of the invention is establishing wheel speed. This has been done in the past by mounting wheel speed sensors at the wheels. As the wheels are free for vertical movement within the limits provided by the suspension, it is inconvenient to mount wheel speed sensors at the wheels.
A recommended type of sensor for determining wheel speed is a new type of inner CV joint type wheel speed sensor which is preferred over existing multi-toothed wheel disk type speed sensors now in common use for xe2x80x9cabsxe2x80x9d braking. As depicted in U.S. Pat. No. 6,082,195, the contents of which are adopted herein by reference, the wheel speed of wheels driven from a trans-axle transmission is measured by sensing the rotational speed of the axle drive shaft as it emerges from the trans-axle transmission case.
Typically, the shaft extending from the trans-axle case terminates at a constant velocity-CV-joint. A typical configuration for a CV joint provides an outer cylindrical sleeve that is interrupted by cut-out portions. One of these sleeves usually rotates in alignment with the protruding shaft of the trans-axle transmission case. The speed of rotation of this sleeve can be measured by mounting a Hall effect proximity detector or similar speed sensor positioned adjacent to the sleeve at a position where the cut-out portions will rotate proximately past the sensor. As this sleeve turns coaxially with the trans-axle transmission shaft, this sensor can conveniently be mounted on the trans-axle casing, avoiding the cost and inconveniences of mounting the sensor on the wheel assembly and installing a multi-tooth indexing disc inside the wheel assembly.
The foregoing summarizes the principal features of the invention and some of its optional aspects. The invention may be further understood by a description of the preferred embodiments, in conjunction with the drawings, which now follow.